1. Ag Content

1) Factors Controlling Ag Content

The Ag content of electrum is dependent on the chemical states of Au and Ag in oreforming fluids. Great efforts to clarify these states have been made mainly through the solubility experiments of Au and Ag minerals in hydrothermal solutions (e.g., Barnes and Czamanske, 1967; Seward, 1973, 1976a, b, 1983; Barnes, 1978; Shenberger, 1985). According to these studies, several chemical states of Au and Ag in ore-forming fluids are considered to be stable.

Seward (1973) has experimentally revealed that dominant chemical states of Au in epithermal ore-forming fluids and active geothermal waters (e.g., Broadlands in New Zealand) are Au thio-complexes among which Au(HS)₂^- is usually dominant in epithermal ore-forming fluids. It is also possible that AuCl₂^- concentration is higher than Au(HS)₂^- concentration in orc-forming fluids responsible for some epithermal vein-type deposits (Casadevall, 1976). However, Au(HS)₂^- is considered to be more abundant than AuCl₂^- in ore-forming fluids responsible for epithermal Au-Ag vein-type deposits in Japan (Shikazono, 1974c; Hattori, 1975; Ichikuni, 1981; Shikazono and Shimizu, 1987).

Dominant Ag species in epithermal ore-forming fluids may be Ag chloride complexes (e.g., AgCl₂^-; Seward, 1976b). But it is also possible that Ag bisulfide complex (Ag(HS)₂^-) is stable in epithermal ore-forming fluids, considering the high solubility of Ag₂S due to bisulfide complexes in hydrothermal solution (e.g., Melent'yev et al., 1969). Unfortunately, thermochemical data on Ag bisulfide complexes at elevated temperatures of concern have not yet been published.

In ore-forming fluids responsible for Japanese hypo/mesothermal Au vein-type deposits, AuCl₂^- may be dominant among Au species (Shikazono and Shimizu, 1986, 1987), although Au(HS)₂^- might also be stable in low temperature regions (probably below ca. 250°C to 300°C). AgCl₂^- may be dominant in the hypo/mesothermal ore-forming fluids (Shikazono and Shimizu, 1987).

There are several other possible stable chemical states of Au and Ag in epithermal and hypo/mesothermal ore-forming fluids: arsenic, tellurium, selenium, bismuth and antimony complexes. However, thermochemical data on these aqueous species have not yet been obtained or published. Therefore, in this study, they are not taken into account.

If Au(HS)₂^- and AuCl₂^- are predominant among Au aqueous species and AgCl₂^- is predominant among Ag aqueous species, the following reactions can be used to derive the relationship among the Ag/Au ratio of electrum and some physicochemical variables (temperature, ∑Au/∑Ag, a_H2S, a_Cl^-, pH).

Au(HS)₂^- + (Ag) + 2Cl^- + 2H⁺ = AgCl₂^- + (Au) + 2H₂S     (1)
AuCl₂^- + (Ag) = AgCl₂^- + (Au)     (2)

in which (Ag) and (Au) represent Ag and Au components in electrum, respectively.

From equilibrium relations for reactions (1) and (2), a_Ag/a_Au (in which a_Ag and a_Au denote activities of Ag component and Au component in electrum, respectively) is represented by

a_Ag/a_Au = (a_AgCl2^-•a²_H2S)/(K₁•a_Au(HS)2^-•a²_Cl^-•a²_H⁺)     (3)

in which K₁ is equilibrium constant of (1).

a_Ag/a_Au = a_AgCl2^-/K₂•a_AuCl2^-     (4)

in which K₂ is equilibrium constant of (2).

If Au(HS)₂^- and AgCl₂^- are predominant Au and Ag species, the equation (3) can be approximated

a_Ag/a_Au = (∑Ag•a²_H2S)(K₁•∑Au•a²_Cl^-)     (5)

If AuCl₂^- and AgCl₂^- are predominant Au and Ag species, the equation (4) can be approximated

a_Ag/a_Au = (∑Ag)(K₂•∑Au)     (6)

To derive the equations (5) and (6), it is assumed that activity coefficient ratios, γ_AgCl2^-/γ_AuCl2^-, γ_AgCl2^-/γ_Au(HS)2^-, are unity.

It is evident in the equations (5) and (6) that a_Ag/a_Au is represented as a function of temperature, a_H2S, a_Cl^-, pH, and ∑Ag/∑Au. It can he obtained that a_H2S is represented as a function of a_S2, a_O2, a_H2O, and temperature from the following reaction:

H₂S + 1/2O₂ = 1/2S₂ + H₂O     (7)

Therefore, in the equilibrium relation for the reaction (7) and equation (5), it is evident that a_H2O, a_S2, and a_O2 also relate to a_Ag/a_Au.

It is likely that the activity of H₂O liquid generally does not deviate significantly from unity under the hydrothermal condition of concern, considering the salinity of orc-forming fluids estimated from fluid inclusion studies and the relationship betwcen salinity and the activity of H₂O liquid (Helgeson, 1969). Therefore, it is reasonable to consider that a_H2O is not an important factor controlling a_Ag/a_Au.

If electrum is in equilibrium with Ag minerals such as argentite, hessite, and naumannite, the following reactions must be taken into account.

2(Ag) + 1/2S₂ = Ag₂S     (8)
2(Ag) + 1/2Se₂ = Ag₂Se     (9)
2(Ag) + 1/2Te₂=Ag₂Te     (10)

From these reactions, it is obvious that a_Ag/a_Au relates to a_S2, a_Se2, and a_Te2. The a_S2, a_Se2, and a_Te2 are represented as a function of ∑S, ∑Se, ∑Te, pH, ionic strength, and temperature. If electrum is in equilibrium with the other Ag minerals (Ag-As, Ag-Sb, Ag-Bi), a_Ag/a_Au depends on concentrations of As, Sb, and Bi in ore-forming fluids.

The relationship among asa, temperature, and mole fraction of Ag in electrum has been experimentally studied by Barton and Toulmin (1964) and Barton (1980). Based on the relationship they obtained, Shikazono (1985b) estimated temperature of formation for the epithermal Au-Ag vein-type deposits in Japan and obtained a good correlation of this temperature with a homogenization temperature of fluid inclusions. From reactions (8), (9), and (10), it is obvious that the Ag content of electrum depends on the kinds of Ag minerals. The influences of physicochemical variables (temperature, a_H2S, pH, a_Cl^-, a_S2, a_Se2, a_Te2) and mineral assemblage involving Ag minerals on the Ag/Au ratio of electruin are considered below.

2) Temperature

Based on the equations (3) and (4), the relationship between a_Ag/a_Au and temperature was derived, assuming that the values for a_H2S, a_K⁺, pH, and ∑Au/∑Ag are constant (Shikazono and Shimizu, 1987). It is also assumed that aK⁺/aH⁺ is controlled by K-feldspar/ K-mica/quartz assemblage because this assemblage is common in the Au-Ag deposits studied. Shikazono and Shimizu (1987) indicated that (1) the effect of temperature on a_Ag/a_Au is notably large in Au(HS)₂^- and AgCl₂^- dominant solutions and that (2) an incrcase in tcmperaturc causes a decrease in a_Ag/a_Au in Au(HS)₂^- and AgCl₂^- dominant solutions.

In Fig. 5, the Ag/Au ratios of electrum and homogcnization temperatures for each deposit were summarized. It can be scen in Fig. 5 that the Ag content of electrum in a low temperature region is higher than the content in a high temperature region. There arc two interpretations on this relationship between the Ag content of electrum and temperature: (1) the N_Ag/N_Au of electrum increases with decreasing temperature, even if the chemistry of the solution docs not change, and (2) the N_Ag/N_Au of electrum in the lower temperaturc region is higher than that of electrum in higher temperature region, reflected by the change in dominant Au species; Au(HS)₂^- is dominant in lower temperature region; and AuCl₂^- is dominant in higher temperature region. The Ag content and homogenization temperatures plotted in Fig. 5 and other evidences (e.g., ace Fig. 6) may support second possibility. However, more precise thermochemical data on the Au species are clearly necessary to evaluatc the effect of temperature. Especially, thermochemical data on Au chloride complexes and Ag bisulfide complexes must be obtained.

It seems very likely that the concentration of Ag(HS)₂^- in hydrothermal solutions may be high in low temperature (less than about 300°C) and low salinity fluids, compared with Ag chloride complexes (Barncs, 1978; Henley, 1985; Brown, 1986). When it is, the following reaction has to be considered:

Ag(HS)₂^- + Au = Au(HS)₂^- + Ag     (11)

Reaction (11) indicates that the Ag/Au ratio of electrum depends on temperature and ∑Ag/∑Au in Ag(HS)₂^- and Au(HS)₂^- dominant solutions.

Henley (1985) and Brown (1986) suggested that Ag bisulfide complex is important for controlling Ag concentration in low salinity (probably less than 0.1 weight % chloride concentration) and near neutral geothermal water, such as Broadlands geothermal water (New Zealand). If their suggestion is correct, the Ag/Au ratio of low salinity geothermal water is controlled by the reaction (11).M

3) Salinity

Differentiating the equation (3) with respect to a_Cl^- we obtain

∂[log(a_Ag/a_Au)]/∂(log a_Cl^-) = -2     (12)

Therefore, in the Au(HS)₂^- and AgCl₂^- dominant region, the Ag content of electrum decreases with increasing a_Cl^- where other variables are constant. However, it is inferred that dominant aqueous Au and Ag species change with the changing of a_Cl^-. For instance, with increasing a_Cl^-, Au(HS)₂^- may change to AuCl₂^-, and AgCl may change to AgCl₂^- with increasing a_Cl^-. In the AuCl₂^- and AgCl₂^- dominant regions, a_Ag/a_Au does not depend on a_Cl^-.

In the natural chloride-rich hydrothermal solutions, m_Cl^- is related to pH (Shikazono, 1978a). Shikazono (1978a) has derived the relationship between m_Cl^- and pH, assuming the equilibrium among common alteration minerals (quartz, K-mica, K-feldspar, chlorite, albite) and chloride-rich hydrothermal solution. In the region in which monovalent cation (alkali element) concentration exceeds divalent cation (alkali earth element) concentration, pH is related to m_Cl^- (in logarithmic unit) with a slope of approximately -1 on a pH-log m_Cl^- diagram (Shikazono, 1978a). In a divalent cation (alkali earth element) dominant region, the slope is approximately -2. Because of this relationship between pH and m_Cl^-, it is derived that the slope of log (a_Ag/a_Au) vs. log a_Cl^- is not -2, when pH is represented as a function of a_Cl^-. The relationship between pH and m_Cl^- is derived, based on the equilibrium between alteration minerals and hydrothermal solution, However, there are several important processes causing the deviation from the mineral/water equilibrium (mixing, boiling, conductive cooling of fluids; Giggenbach, 1981; Shikazono, 1985c).

Figure 6 summarizes the relationship between the Ag content of electrum and the salinity (NaCl equivalent concentration) of fluid inclusions. It can be seen in Fig. 6 that the Ag content of electrum precipitated from lower salinity fluids is higher than that precipitated from higher salinity fluids. For instance, epithermal ore-forming fluids have low salinity, but Kuroko, hypo/mesothermal and skarn. ore-forming fluids have high salinity, and Ag contents of electrum from these deposits are lower than those from epithermal deposits, This observed evidence seems consistent with the theoretical prediction discussed above. Therefore, it is most likely that the Ag content of electrum is controlled by a chloride concentration in hydrothermal solution.

4) pH

It is expected from equation (3) that the Ag content of electrum decreases with decreasing pH of the solution in which dominant Au and Ag species are Au(HS)₂^- and AgCl₂^-.

The concentration ratio of mΛu (HS)a-/mAuCla- is determined by the equilibrium relation for the following reaction:

Au(HS)₂^- + 2Cl^- + 2H⁺ = AuCl₂^- + 2H₂S     (13)

From the equilibrium relation for reaction (13), it is derived that mΛuCla-/mΛu(IIS)a- increases with decreasing pH. Thus, at lower pH condition, AuCl₂^- concentration becomes higher than Au(HS)₂^- concentration. In AuCl₂^- and AgCl₂^- dominant conditions, the Ag/Au ratio of electrum does not depend on pH.

In epithermal Au disseminated-type deposits in Japan (Kasuga, Iwato), alteration minerals such as kaolinite, sericite, alunite, and pyrophyllite are commonly observed, and the Ag/Au ratio of electrum is very low (Urashima et al., 1981a). In the Chinkuashih and Chiufen deposits in Taiwan, advanced argillic and argillic alterations, including kaolinite, alunite, and sericite, are associated with Au-Ag mineralization. The Ag content of electrum from these deposits ranges from 31.2 to 44.0 atomic %, which is usually lower than for typical epithermal Au-Ag vein-type deposits in Japan and the Korean Peninsula.

Adularia occurs commonly in epithermal Au-Ag vein-type deposits in Japan. Sericite is common in Kuroko-type deposits and hypo/mesothermal vein-type deposits in the Korean. Peninsula (Kaneda et al., 1986). At constant a_K⁺, a_SiO2, and temperature, sericite is stable under a lower pH condition than. that for K-feldspar, In general, the Ag content of electrum from Kuroko-type and hypo/mesothermal vein-type deposits is lower than that from epithermal Au-Ag vein-type deposits. Therefore, the relation between the kinds of gangue and alteration minerals and the Ag content of electrum seems to be consistent with the theoretical consideration mentioned above. This suggests that pH is an important factor in determining the Ag content of electrum.

5) Activities of H₂S, S₂, and O₂

From equation (3) it is inferred that a_Ag/a_Au increases with increasing a_H2S. However, an evaluation of influence of a_H2S on the a_Ag/a_Au ratio is difficult because of the difficulty of an estimate of a_H2S in ore-forming fluids. The relationship among a_S2, a_O2, and a_Ag/a_Au in Au(HS)₂^- and AgCl₂^- dominant ore-forming fluids can be obtained from equation (3) and the equilibrium relation for the following reaction.

H₂S + 1/2O₂ = 1/2S₂ + H₂O     (14)

In Fig. 7, the Ag content of electrum and the FeS content of sphalerite from several types of deposits are summarized. Figures 7-A, 7-B, and 7-C reveal that epithermal Au-Ag vein-type deposits are characterized by a high Ag content of electrum, and a low FeS content of sphalerite and that hypo/mesothermal vein-type deposits and skarn-type deposits are characterized by a low Ag content and a high FeS content. The FeS content of aphalerite from Kuroko-type deposits is low, less than 1 mole % (Shikazono, 1974c), and the Ag content is variable. The FeS content of sphalerite coexisting with iron minerals (pyrite and pyrrhotite) relates to a_S2, (Barton and Toulmin, 1966). Thus, it is likely that a_S2 for hypo/mesothermal vein-type deposits is different from that for epithermal Au-Ag vein-type deposits (Shikazono and Shimizu, 1987). Estimates of a_S2 and temperature for epithermal and hypo/mesothermal vein-type deposits are carried out in Chapter VI. A negative correlation between the FeS content of sphalerite and the Ag content of electrum as shown in Fig. 7 can be explained by the Ag content of electrum from hypo/mesothermal vein-type and skarn-type deposits not being controlled by the equilibrium relation for reaction (8); electrum from these deposits does not coexist with argentite.

6) Minerals Coexisting with Electrum

Figure 8 illustrates the relationship between the Ag content of electrum and the Ag-Au total production ratio from an individual deposit. Shikazono (1986) noted that a positive correlation between the total production ratio of Ag to Au and the Ag-Au ratio of electrum in a given ore deposit seems to exist. The epithermal base metal-rich deposits have a high Ag-Au total production ratio and a high Ag-Au ratio of electrum. In contrast, Au-Ag-rich and base metal poor epithermal deposits have a relatively low Ag content of electrum and a low Ag-Au total production ratio.

It is worthy to note that argentite occurs abundantly in the ore deposits with a high Ag-Au total production ratio, and argentite is poor or absent in the ore deposits having a low Ag-Au total production ratio. This implies that if argentite precipitates in deposits, the Ag content of electrum coprecipitating with argentite is high and the Ag-Au total production ratio of deposits becomes higher. In the deposits with low Ag-Au total production ratio, dominant Ag minerals are Ag sulfosalts (polybasite and pyrargyrite), Ag-Se-S minerals (naumannite, aguilarite, Se-bearing argentite), hessite, and Ag-bearing tetrahedrite. The amounts of these Ag minerals precipitated in a given ore deposit are usually small.

Figure 9 summarizes the Ag content of electrum from Cu-rich and Cu-poor deposits. It can be seen in Fig. 9 that the Ag content of electrum from Cu-rich deposits is lower than that from Cu-poor (Pb-Zn-rich) deposits. This relation exists in Kuroko-type and epithermal base metal vein-type deposits. The Ag content of electrum from the siliceous and yellow ores of Kuroko-type deposits in which Cu is rich is lower than that from the black ore in which Pb and Zn are rich but Cu is poor. The dominant Cu minerals in these deposits are chalcopyrite and bornite. Although Cu is not produced from the sulfide-poor deposits (epithermal Au disseminated-type, alaskite-type, and hypo/mesothermal Au veintype deposits), Cu minerals (chalcopyrite, enargite, luzonite) are occasionally found in them, but Pb, Zn, and Ag minerals are poor.

Table 9 shows that the Ag content of electrum coexisting with Bi and/or Te-bearing minerals is relatively low. The main Bi-Te-bearing minerals are hessite and petzite in epithermal Au-Ag vein-type deposits, and hessite, tellurobismuthite, tetradymite, and matildite in hypo/mesothermal vein-type deposits.

It is evident in Table 10 that the Ag content of electrum coexisting with pyrrhotite and arsenopyrite is relatively low. The types of deposits in which pyrrhotite and arsenopyrite commonly occur are hypo/mesothermal vein-type, skarn-type, and alaskite-type deposits. In the other types of deposits, these minerals are rare. The main As-bearing minerals in the other types are tctrahedrite-tennantite (Kuroko and epithermal Au-Ag vein-type) and luzonite and enargite (epithermal Au disseminated-type).

By comparing Figs. 10 and 11 with Table 9 the Ag content of electrum coexisting with argentite, naumannite, and aguilaritc is seen to be highcr than that of electrum coexisting with hessite. Therefore, a difference in Ag minerals coexisting with electrum relates to different Ag contents of electrum, although the reason is still uncertain.

Figure 12 shows the Ag content of electrum from the deposits in which K-feldspar commonly occurs. In general, the Ag content of electrum is high, more than 40 atomic %, On the other hand, the Ag content of clectrum from the deposits in which sericite (muscovite) is common is low (Fig. 13), compared with the Ag content from deposits in which K-feldspar occurs.

7) Thermochemical Mixing Properties

To apply equations (3) and (4) to natural electrum, the activity-composition relationship in Au-Ag alloy must be known. The thermochemical mixing properties of Au-Ag alloy have been studied by White et al. (1957). Based on this study, Barton and Toulmin (1964) successfully derived a_S2-temperature-N_Ag relation in an electrum-argentitc-S₂ vapor system, Figure 14 shows the variations in γ_Ag/γ_Au (γ_Ag: activity coefficient of Ag, γ_Au: activity coefficient of Au) as a function of N_Ag calculated from the thermochemical data by White et al, (1957). It can be seen in Fig. 14 that γ_Ag/γ_Au is close to unity at N_Ag=0.5, but this ratio deviates significantly from unity toward higher and also lower N_Ag valucs.

2. Contents of Other Elements

Hg content:

If electrum is in equilibrium with cinnabar, the following reaction can be used to obtain the relationship among the Hg content of electrum, asa and temperature.

(Hg) + 1/2S₂ = HgS     (15)

in which (Hg) is the Hg component in electrum.

From the equilibrium for (15), it is obvious that a_Hg is represented as a function of temperature and a_S2 and that a_Hg increases with decreasing a_S2 at a given temperature, Shikazono and Shimizu (1988) have reported a high Hg content of electrum from the Tsugu Au-Sb-Hg vein-type deposit (Hg: up to 7.7 atomic %) and indicated that electrum having a high Hg content occurs in Au deposits in which the opaque minerals indicating low a_S2 conditions (pyrrhotite, cubanite) occur. This occurrence of opaque minerals is consistent with a high Hg content of electrum from. these deposits.

If HgS is not present, a_S2 for electrum depends on the chemical states of Hg in oreforming fluids. In this case, a_Hg depends on the factors (e.g., ∑Hg in ore-forming fluids) other than a_S2 and temperature. There are several possible Hg species in orc-fornaing fluids (e.g., Hg bisulfide complex and organic complexes) and a transport of Hg by vapor is also probable (Barnes, 1978).

Cu content:

The Cu content of electrum coexisting with copper and iron minerals (chalcocite, chalcopyrite, pyrite, pyrrhotite) is determined by the following reactions.

2(Cu) + 1/2S₂ = Cu₂S     (16)
(Cu) + S₂ = CuS₂     (17)
5(Cu) + FeS₂ + S₂ = Cu₅FeS₄     (18)
(Cu) + FeS₂ = CuFeS₂     (19)
(Cu) + FeS+1/2S₂ = CuFeS₂     (20)

in which (Cu) is the Cu component in electrum.

The reactions (16) to (20) indicate that a_Cu (activity of Cu component in electrum) epends only on temperature in the pyrite and chalcopyrite stable region, whereas it depends on a_S2 and temperature in the bornite, chalcocite, covellite, and pyrrhotite stable regions. In these regions, a_Cu increases with decreasing a_S2 at a given temperature. Yamaoka (1981) reported high Cu content of electrum from the Kamaishi skarn deposit, Cuprian electruni from the Kamaishi deposit coexists with pyrrhotite. Thus, it is inferred that cuprian gold formed under a relatively low a_S2 condition. However, data on cuprian eleetrum) are scarce, and a mode of occurrence of cuprian electrum is not well known. Further works on the Cu content of electrum arc required.

Sb content:

The Sb content of electrum in equilibrium with stibnite is controlled by the following reaction:

2(Sb) + 3/2S₂ = Sb₂S₃     (21)

in which (Sb) is the Sb component in electrum.

Reaction (21) implies that a_Sb increases with decreasing a_S2 at a given temperature (Fig. 15). Under the low a_S2 condition, gold reacts with stibnite to form aurostibite. It was reported by Urashima et al. (1981) that the Sb content of electrum from the Sakoshi-Odomari deposit is high (up to 2.4 weight %). The activity of S₂ for the Sakoshi-Odomari mineralization is estimated to be relatively low because pyrrhotite is found and the FeS content of sphalerite is high (5.6 to 7.9 atomic % Fe; Urashima et al., 1981b), compared with the typical Fe content of sphalerite from epithermal Au-Ag vein-type deposits (Shikazono, 1977b). Low a_S2 condition is consistent with high Sb content of electrum from the Sakoshi-Odomari deposit. The thermochemical mixing properties of Au-Ag-Sb alloy must bc studied to estimate the a_S2-temperature range from the Sb content of electrum.